Inflight, most of the outside air that gets compressed in the aircraft engines is used for thrust. But with the exception of the Boeing 787, the air supply systems are designed to extract (or “bleed”) a portion of that high-temperature, high pressure air off the engine compressors, cool it, condition it, mix it with recirculated air, and route it to the cabin and flight deck for ventilation air (view schematic). During ground operations when the aircraft engines are turned off, the ventilation air may be supplied from a variety of sources, including the aircraft auxiliary power unit, which is a small engine in the tail of the aircraft. Because the ventilation air is “bled” off an engine, it is called “engine bleed air.”

Engineering reports dating back to the 1950s acknowledge the potential for engine oil smoke/fumes to contaminate the bleed air routed to the aircraft cabin and flight deck. Causes of contamination include failed engine seals, ruptured or leaking oil supply lines, high stress on engine components during transient power settings, and maintenance staff overfilling an oil reservoir, for example. Because the bleed air is not filtered, oil smoke/fumes in the bleed air directly contaminate the cabin air, flight deck air, or both, depending on the source of the contamination. Aviation regulators, including the US Federal Aviation Administration,  have recognized the potential for airline crews to be exposed to oil smoke/fumes during commercial flights, but currently bleed air supply systems are not equipped with either chemical sensors or filtration systems.

Aviation engine oils used on the commercial airline fleet contain a complex mixture of chemical compounds, many of which are proprietary. Most aviation engine oils are formulated with a pentaerythritol base stock, although at least one (BP2380) is formulated with a trimethylol propane phosphate base stock. All oils used on the commercial airline fleet contain tricresyl phosphates (TCPs), according to the hazardous ingredient information listed on most product safety data sheets (SDS) and also identified in published chemical analyses of the engine oils and associated fumes. The SDS for some engine oil products report the presence of phenyl naphthylamine and trixylenyl phopshates. Other chemical compounds identified in oil fumes include, but are not limited to acetaldehyde, acrolein, carbon monoxide (temperature-dependent), carboxylic acids, formaldehyde, phthalates, toluene, and xylene. The complexity of the chemical mixtures in the oil fumes, combined with the potential for inter-individual susceptibility to ill effects and variable ambient pressure inflight, preclude the ability to define a “no adverse effect level” for exposure to engine oil fumes on aircraft.